i'm curious as to why polycarbonate lenses are stronger and roughly the same weight as hi-index, but hi-index and mid-index lenses costs more? my understanding are that hi-index lenses are 1.6-1.7 index, poly are 1.6, the usual lenses are 1.5, and mid index lenses are about 1.5+. if so, why are the mid index lenses more expensive than the polycarbonate? i've tried out polycarb lenses a couple of years ago and i noticed a yellowish effect at the edges of white objects. is this distortion why poly lenses are less expensive? i'm gonna try some mid-index lenses for my next pair of glasses. just wanted some input.